Oil-filled electrical apparatus



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I I/ //I [T Aug. 13, 1946. R CH 2,405,853

OIL-FILLED ELECTRICAL APPARATUS Filed May 29, 1942 ENVENTOR- SAMUEL J. RoscH ATTORNEYS Patented Aug. 13, 1946 OIL-FILLED ELECTRICAL APPARATUS Samuel J. Bosch, Yonkers, N. Y., assignor to Anaconda Wire and Cable Company, a corporation of Delaware Application May 29, 1942, Serial No. 445,007

8 Claims.

This invention relates to electric apparatus such as transformers in which transformer windings or other electrical equipment is immersed in oil within a tank or case, and has for its principal object the provision of means for minimizing deterioration of oils in such electric apparatus.

Electric transformers, except very small electric transformers, generally have the transformer windings mounted in a liquid tight case (or tank). The case is filled with an insulating oil which is caused to circulate (either by thermallyinduced currents or by mechanical circulating means such as a pump) to effect cooling of the windings, and the oil also provides an insulating medium between these windings. The most satisfactory cooling and insulating oils are petroleum oils which have been highly refined to remove constituents which would otherwise lower the dielectric strength of the oil. Such constituents include moisture, acidic compounds, sludge particles and entrapped air. The resulting finished insulating oil possesses initially high dielectric strength, but the conditions to which it is subjected in use promote deterioration of the oil which results in lowering its dielectric strength. For example, it is not usually practical to make a. transformer case completely airtight, as the need for expansion room required due to fluctuating temperatures of the insulating oil generally requires the provision of breather openings in the case. The oil thus is allowed to come in contact with air, which effects oxidation of the oil with formation of acidic constituents and deterioration products known as sludge. The relatively high temperatures to which the oil becomes heated in normal operation of transformers and the like accelerate this oxidation of the oil. Furthermore, oxidation is catalyzed by the metallic iron and copper in contact with the oil within the transformer, and reaction which takes place between acidic constituents and the metallic iron and copper yields reaction products which further catalyze oxidation and deterioration of the oil.

The acidic constituents formed in transformer insulating oils, in addition to promoting further deterioration of the oil, are of low dielectric strength and may attack the insulating material about the transformer windings. These factors, together with the accumulation of sludge in the oil, are likely to lead to electrical break-down within the transformer unless the oil is replaced at proper intervals or unless some provision is made to retard deterioration of the oil. To the latter end, anti-oxidants, or sludge inhibitors, are sometimes added to the oil. Such addition agents are relatively expensive and many of them have the disadvantage that they increase the attack upon the metals of the transformer, or they break down at the high operating temperatures such as commonly result from imposing heavy loads on the transformer.

It has been proposed to minimize deterioration of transformer oils by circulating the oil in contact with loose carbon black in a suitable container. This expedient has not proven commercially satisfactory because it is impossible practically to prevent a proportion of the fine particles of carbon black from entering suspension in and circulating with the oil. When this .occurs the deleterious components which the carbon black is supposed to remove still circulate with the oil, come in contact with the iron of the case and core and the copper of the conductors,

and tend to accumulate in voids, cracks, and

other locations in much the same manner as if the carbon black were not employed. As a result deterioration of the oil is not prevented, even though it may be somewhat retarded. Moreover, carbon black is an electrical conductor, and the tendency of loose particles of carbon black and sludge or other contaminants to accumulate in small voids of the winding insulation enhances the hazard of electrical break-down within the transformer.

In the course of extensive investigation of means for minimizing deterioration of transformer insulating oils, I have discovered that carbon black incorporated in and held in place by a fibrous material has the desirable characteristic of inhibiting oxidation of such oil and of removing (probably by preferential absorption) deterioration products such as sludge, so that deterioration of the oil is substantially prevented over prolonged periods. Based upon this discovery, the present invention provides the combination, with an electric transformer comprising a case having therein transformer windings immersed in oil, of a cartridge comprising fibrous material (advantageously fibrous sheet material) having carbon black incorporated therein, the cartridge being disposed within the transformer case in contact with the oil. Preferably the cartridge is readily replaceable so that it may be removed from time to time, as may be required, to maintain the oil in the best possible condition.

The fibrous material may with advantage be paper, and the carbon :black is advantageously 3 incorporated in the pulp during manufacture of the paper so that it is thoroughly impregnated and firmly held in place therein.

In the usual oil-filled transformer the oil serves as a cooling medium, and is caused to circulate (by thermal currents or mechanical pumping means) upwardly in close proximity to the windings and downwardly in close proximity to the walls of the case (or other heat-radiating surface). It is advantageous to dispose the cartridge of fibrous material impregnated with carbon black in the path of oil circulation so that the oil flows in contact with the arbon black and the entire body of oil is thus kept free of deleterious contaminants. The cartridge ma be disposed either in the path of upward circulation or in the path of downward circulation whichever is the more convenient. If desired, a plurality of cartridges disposed in the path of upward or downward circulation, or both, may be employed.

What has been said above with respect to the deterioration of transformer oils and the improvements according to the invention for preventing such deterioration, is true also of oils in other oil-filled electrical apparatus such as oil- I filled switches or circuit-breakers, oil-filled rheostats, and other electrical apparatus comprising a case having therein electrical equipment immersed in oil. The invention therefore contemplates the provision in all such apparatus of a replaceable cartridge comprising fibrous material having carbon black incorporated therein, the cartridge being disposed within the case in contact with the oil, whereb the oil is preserved from deterioration and its initially high dielectric strength is substantially maintained.

The invention is described in greater detail below with particular referen e to the embodiments shown in the accompanying drawing, in which Fig. l is a longitudinal section through a coretype transformer showing replaceable cartridges according to the invention mounted therein;

Fig. 2 is a cross section taken substantially along the line 2-2 of Fig. 1;

Fig. 3 is a longitudinal section through a container for one form of cartridge according to the invention;

Fig. 4 is a plan of the container shown in Fig, 3, with cover removed to show the cartridge in place;

Fig. 5 is a fragmentary perspective of a modified form of cartridge; and

Fig, 6 is a cross section through a modified shell-type transforme showing a convenient location of replacable cartridges according to the invention.

Figs. 1 and 2 show in section, more or less diagrammatically, an oil-filled core-type transformer. The transformer comprises a case or tank iii to which a cover l i may be bolted. Within the tank or case are the transformer windings l2 about the usual iron core is. The tank is filled with a good ualit of insulating oil to a level L so that the transformer elements ar completely immersed. Fibrous material impregnated with carbon black and put up in the form of replaceable cartridges is disposed within the transformer case below the level L of the oil. One type of cartridge is held in a perforated container i i. As shown in the drawing, two such containers it are supported at the top of the core is by a bracket it. The con- 4 vantageously are located, as shown in the drawing, in the path of upwardl circulating oil risin between the transformer windings l2.

A form of container and cartridge of fibrous material impregnated with carbon black which may be mounted conveniently within the transformer is shown in Fig. 3. This container comprises a perforated body portion 56 having a perforated cover ll. The perforations are sufficiently lar e and closely spaced so that oil may circulate through the container freely. A mounting bracket 18 is affixed to the container body so that it may be attached to any convenient support within the transformer". Disposed within th container i5 is the cartridge E9 of fibrous material impregnated with carbon black. A convenient form of cartridge l9 (shown best in Fig, 4:) comprises a sheet of fibrous material such as paper which has been tho-roughly impregnated with carbon black and wound into the form of an open spiral, having an outer diameter such that it may easily be slipped into or removed from the container 16. A spider 26 may be arranged at each end of the spirall wound sheet of fibrous material to maintain it permanently in its loose spiral form. The spirally wound cartridge presents a large surface to contact with the oil, and since the spiral is wound loosely, oil circulates freely between the successive convolutions. If desired, the fibrous material may itself be perforated so that the oil may circulate laterally through the cartridge as well as vertically between the successive convolutions.

A modified type of cartridge 2| is also shown mounted in the transformer of Figs. 1 and 2. This form of cartridge is roughly in the form of a short cylinder of sufiicient internal diameter so that it may be passed over the transformer windings l2. It may, of course, be other than truly cylindrical and is shown in Fig. 2 as being somewhat squared. This form of cartridge may be hung on hooks 2la secured to the wall of the transformer case. The construction of the cartridge 2| is shown more in detail in Fig. 5. It comprises a pair of ring-form supporting elements 22 held in spaced relation by generally U-shaped brackets 23 located at intervals about the ring. The supporting rings 22 are formed with grooves 24 which face each other, and held in the groove are the edges of successive convolutions 25 (arranged either spirally or concentrically, as desired) of fibrous sheet material impregnated with carbon black. The supportiing rings are liberally perforated, preferably between the grooves 24, so that oil may circulate between the successive convolutions of the sheet fibrous material held in place by the rings. One or both of the rings 22 advantageousl is provided with a bracket 28 for mounting it in the transformer.

The two forms of cartridge described above are merely examples of suitable cartridges for use in accordance with the invention. Many other mechanical forms of cartridges may be designed by one skilled in the art. For example, the cartridge may comprise a single sheet or strip of fibrous material impregnated with carbon black positioned, for example, about the transformer windings (or other electrical equipment), the sheet or strip being spaced advantageously from both the windings and the case. In all such cartridges, however, it is desirable that a substantial area of the fibrous material impregnated with carbon black be exposed to the oil, and the form of the cartridge or its container surface. tainers 14 for the carbon black impregnated fibrous cartridges are mounted so that it is in purpose is paper.

Should be such that the oil is able to circulate freely in contact therewith.

The location of the cartridge within the transformer case is not particularly critical so long as it is in good contact with the oil, but for most effective results it is desirable that the cartridge be placed in the path of circulation of the oil so that the oil is continuously flowing in contact with the fibrous material. In the case of transformers where the oil circulation is thermally induced, the oil flows upwardly in close proximity to the transformer windings and core and downwardly in close proximity to the walls of the case or other heat radiating As shown in Figs. 1 and 2, the conthe path of oil circulating upwardly in close proximity to the windings l2 and the core. The cartridge 23 is so positioned as to be in the path of oil circulating downwardly in close proximity to the walls of the case [0.

The particular location of the cartridges within the transformer case also depends to some extent on the design and construction of the particular transformer. It is customary in coretype transformers to have terminal posts for the windings arranged on an insulating support mounted at the top of the core l3. In such a transformer the containers 14 may have to be displaced somewhat from the position shown in Figs. 1 and 2 in order to make room for the terminal posts and the supporting structure therefor. Other types of transformers may require mounting of the containers in other locations. For example, Fig, 6 shows in cross section a modified shell-type transformer comprising a case 39 filled with oil in which the transformer unit is immersed. This unit comprises a core 31 which is X-shaped in plan. The core 3| has a central leg about which coils 32 are disposed and peripheral legs outside the coils for completing the magnetic path. In a transformer of this sort, cartridge containers 33 according to the invention may be mounted at the extremities of the peripheral legs of the core, either on the core itself or on any convenient supporting structure within the transformer case below the level of the oil therein.

' It is desirable that the cartridge (or the cartridge container) be fairly accessible so that if required from time to time an old cartridge may be removed easily and a new cartridge sub- .stituted therefor in order to maintain the dielectric strength of the oil at a high value for prolonged periods of time.

Any fibrous material into which carbon black may be impregnated may be employed in making up the cartridge. However, it is advantageous to have the fibre-us material in sheet form so as to present a relatively large surface per unit of volume to the oil. The most inexpensive and generally satisfactory fibrous material for this The carbon black is readily and completely incorporated in paper by beating it with the pulp from which the paper is made. The resulting. carbon black paper is highly effective for the purposes of the invention, and firmly retains the carbon black in place, so that no free particles of carbon black are allowed to enter and circulate with the oil.

Any suitable weight of paper may be employed, depending upon the mechanical structure of the cartridge itself. A cartridge designed to be held in a container, such as is shown in Figs. 3 and 4,

may be of ordinary kraft paper of the weight and quality usually employed as electrical insulation. For a cartridge of the type shown in Fig. 5, on the other hand, the carbon black paper should be of rather heavy weight so as to have adequate inherent stiffness to retain its form and resist sagging or buckling when wetted by the oil in which it is to be immersed.

In referring herein to carbon black, I mean particularly finely divided carbon produced as a result of imperfect combustion of natural gas or similar hydrocarbon gases. This carbon black (by which name it is known to the trade) is to be distinguished from lamp black, for example (produced by the imperfect combustion of carbonaceous oils or oily liquids), and other forms of carbonaceous blacks or carbons produced in other ways from other materials. Such forms of carbon as lamp black or graphite, for example, are quite incapable of maintaining transformer or other electrical oil at its initially high dielectric strength, and may even detract therefrom. A carbon black of the grade known to the trade as Micronex i highly satisfactory. Such carbon black advantageously constitutes about 10% by weight of the paper or other fibrous material from which the cartridge is formed.

Although the invention has been described above with particular reference to oil-filled transformers, it is of course understood that the replaceable cartridge of the invention may be employed with advantage in other oil-filled electrical apparatus in which the oil is subject to deterio'ration after use for any considerable period of time. For example, suitable cartridges comprising fibrous material impregnated with carbon black are readily designed for use in such oilfilled electrical apparatus as oil-filled switches, oil-filled circuit breakers, oil-filled rheostats, and the like.

I claim:

1. A cartridge for immersion in the insulating oil used in transformers to absorb deterioration products from the oil which comprises a supporting structure, a plurality of spaced sheets of fibrous material having carbon black incorporated therein, said sheets being so spaced that substantially the entire surface of each of the sheets is exposed to contact with the oil, means for securing the sheets in said spaced positions in the supporting structure whereby the oil may flow between the sheets without passing therethrough, the space between said sheets being substantially free of encumbrance to the flow of oil therethrough and passage means in the supporting structure through which the oil may flow and pass into the space between the spaced sheets in the course of normal circulation of the oil in the transformer.

2. A cartridge for immersion in the insulating oil used in transformers to absorb deterioration products from the oil which comprises a supporting structure, a plurality of spaced sheets of fibrous material having carbon black incorporated therein, said spaced sheets being in the form of a spiral and being so spaced that substantially the entire surface of each turn of the spiral sheet is exposed to contact with the 011, means in the supporting structure for securing the sheets in said spaced positions substantially free of encumbrance to the flow of oil therethrough, and passage means for permitting the flow of oil through the cartridge and in the spaces between the sheets without passing through the sheets in the course of normal circulation of the oil in the transformer.

3. A cartridge for immersion in the insulating oil used in electric apparatus in which electrical equipment is immersed in oil, the cartridge being adapted to absorb deterioration products from the oil and comprising a supporting structure, a plurality of spaced sheets of fibrous material having carbon black incorporated therein, said sheets being so spaced that substantially the entire surface of each of the sheets is exposed to contact with the oil, means for securing the sheets in said spaced. positions in the supporting structure whereby the oil may flow between the sheets without passing therethrough, the space between said sheets being substantially free of encumbrance to the flow of oil therethrough, and passage means in the supporting structure through which the oil may fiow and pass into the space between the spaced sheets in the course of normal circulation of the oil in the apparatus.

4. A cartridge capable, upon immersion in the insulating oil in oil-filled electrical apparatus, of absorbing deterioration products from the oil by contact between the oil and the cartridge as the oil flows between contact elements in the cartridge in the course of normal circulation of the oil within the apparatus, the cartridge comprising a supporting structure, a plurality of contact elements in the form of spaced sheets of fibrous material having carbon black incorporated therein, said sheets being so spaced that substantially the entire area of each of the sheets is exposed to contact with the oil flowing therebetween, means for securing the sheets in such spaced positions in the supporting structure that the oil may flow between the sheets without passing through the sheets, the space between said sheets being substantially free of encumbrance to the flow of oil therethrough, and passage means in the supporting structure through which the oil may flow and pass into the space between the sheets in the course of normal circulation of the oil'in the oilfilled apparatus.

5. In an electric transformer having a case within which are arranged transformer windings immersed in oil, the improvement which cornprises a replaceable cartridge capable of absorbing deterioration products from the oil by contact between the oil and the cartridge as the oil flows between contact elements in the cartridge in the course of normal circulation of the oil within the transformer, the cartridge comprising a supporting structure, a plurality of contact elements in the form of spaced sheets of fibrous maerial having carbon black incorporated therein,

said sheets being so spaced that substantially the 6. A transformer comprising a transformer core and windings mounted in a tank, oil filling the residual space in said tank and circulating therein, a replaceable cartridge serving to remove deterioration products from the oil removably mounted within the tank, said cartridge comprising a supporting structure, a plurality of contact elements in the form of spaced sheets of fibrous material having carbon black incorporated therein, said sheets being so spaced that substantially the entire area of each of the sheets is exposed to contact with the oil flowing therebetween, means for securing the sheets in such spaced positions in the supporting structure that the oil may flow between the sheets without passing through the sheets, the space between said sheets being substantially free of encumbrance to the flow of oil therethrough, and passage means in the supporting structure through which the oil may freely flow and pass between and in contact with substantially the full area of both surfaces of the sheets during the course of its normal circulation within the transformer, said cartridge being so positioned within the tank that th passages through the supporting structure and between the sheets thereof is in line with the path taken by the oil in the course of its normal circulation within the tank.

7. A cartridge for immersion in the insulating oil used in electric apparatus in which electrical equipment is immersed in oil, the cartridge being adapted to absorb deterioration products from the oil and comprising perforated end members having disposed therebetween a plurality of spaced sheets of fibrous material having carbon black incorporated therein, said sheets being so spaced that substantially the entire surface of each of the sheets is exposed to contact with the oil, the space between said sheets being substantially free of encumbrance to the flow of oil therethrough and the perforation in said end members being so positioned with respect to the spaced sheets as to permit the oil to pass into the space between the sheets and to flow between the sheets without passing therethrough in the course of normal circulation of the oil in the apparatus.

8. A cartridge for immersion in the insulating oil used in electric apparatus in which electrical equipment is immersed in oil, the cartridge being adapted to absorb deterioration products from the oil and comprising a plurality of spaced sheets of fibrous material having carbon black incorporated therein, and perforated end members supporting the sheets in spaced position, said sheets being so spaced that substantially the entire surface of each of th sheets is exposed to contact with the oil, the space between the sheets being substantially free of encumbrance to the flow of oil therethrough and the perforations in said end members being so positioned with respect to the spaced sheets as to permit the oil to pass directly into the space between the sheets and to flow between the sheets without passing therethrough in the course of normal circulation of the oil in the apparatus.

SAMUEL J. BOSCH. 

